1. Field of the Invention
The present invention relates to a packaging material for a battery, which has moisture barrier properties and resistance to contents of the battery and is usable for batteries with a liquid or solid organic electrolyte (polyelectrolyte), fuel batteries (cells), capacitors and the like.
2. Background Art
The term “battery” as used in the present invention refers to objects including devices for converting chemical energy to electric energy, for example, lithium ion batteries, lithium polymer batteries, and fuel batteries, or electrolytic capacitors containing liquids, solid ceramics, organic materials or other dielectric materials, for example, liquid capacitors, solid capacitors, and bilayer capacitors.
Batteries are used for applications such as personal computers, portable terminal devices (for example, portable telephones (cellular phones) and PDA), video cameras, electric automobiles, energy storage batteries, robots, and satellites.
Metal cans produced by pressing metals to form cylindrical or rectangular parallelepiped vessels, or bags formed from a laminate of a composite film produced by lamination of plastic films, metal foils and the like (hereinafter referred to as “armor body”) have hitherto been used as the armor body for the above batteries.
The conventional armor bodies for batteries, however, have the following problems. In metal cans, since the outer wall of the vessel is rigid, the shape of the battery per se is disadvantageously limited. For this reason, since the hardware side is designed according to the battery, the dimension of the hardware used in the battery is disadvantageously determined by the battery and, thus, the degree of freedom in shape is reduced.
For this reason, the above bag-like armor body is generally used. From the viewpoints of properties required of batteries, moldability or fabricability, profitability and the like, the material for the armor body comprises at least a substrate layer, a barrier layer, a sealant layer, and an adhesive layer for bonding the above layers to each other, and an intermediate layer is optionally provided.
A pouch is formed from a laminate having the above construction for a battery, or alternatively at least one side of the laminate is subjected to press molding to form a housing part for a battery. A battery body is housed in the pouch or the housing part, and, in the pouch type or the emboss type (covered with a lid), the necessary part of the peripheral edge thereof is heat sealed for hermetic sealing to prepare a battery.
The innermost layer in the sealant layer should have heat sealing properties between the innermost layers and, in addition, should have heat sealing properties on leads (metals). For example, the use of an acid-modified polyolefin resin having adhesion to metals in the innermost layer can ensure the adhesion to leads.
A. Stacking the acid-modified polyolefin resin as the innermost layer in the armor body, however, is disadvantageous, for example, in that, as compared with general polyolefin resins, the moldability or fabricability is inferior and, in addition, the cost is higher. For this reason, a method has hitherto been adopted wherein a general polyolefin resin layer is used as the innermost layer of the sealant layer in the armor body and a film for a lead, which is heat bondable to both the innermost layer and the lead, is interposed in the lead part.
More specifically, as shown in FIG. 1G (a), a film 6′ for a lead, which has heat sealing properties on both a metal and a sealant layer in the armor body, is interposed between a lead 4 and a sealant layer 14′ in a laminate 10′ to ensure hermetic sealing properties in the lead part.
A film of the above unsaturated carboxylic acid-grafted polyolefin, a metal-crosslinked polyethylene, or a copolymer of ethylene or propylene with acrylic acid or methacrylic acid may be used as the film for a lead.
The sealant layer in the laminate for constituting the armor body for a battery (hereinafter referred to as “armor body”) is formed of polypropylene, for example, from the viewpoints of heat resistance and hermetic sealing properties. In this case, a polypropylene resin, which exhibits good hermetic sealing properties and is likely to be collapsed upon heating and pressing at the time of heat sealing, that is, a polypropylene resin having a large melt index (hereinafter referred to as “MI”), is used. An acid-modified polypropylene film is used as the film for a lead. Upon hermetic heat sealing using the packaging material for a battery having the above construction and the film for a lead having the above construction, as shown in FIG. 1G (b), both the heat seal layer 14′ in the armor body and the film layer 6′ for a lead are melted, at a portion where the lead exists, by heat and pressure for heat sealing and, further, upon pressing, are often extruded to the outside of the region of the pressing part. As a result, the aluminum foil as the barrier layer 12′ in the armor body 10′ often comes into contact (S) with the metal lead 4′, resulting in short-circuiting.
Further, as shown in FIGS. 1H (a) to 1H (c), upon heat sealing of the peripheral edge of the armor body, microcracks (hereinafter referred to as “root cutting C”) often occur in the sealant layer in its portion near the inner edge of the sealed part. Upon root cutting, an electrolysis solution comes into direct contact with the barrier layer. As a result, insulation among the battery body, the metal of the lead, and the barrier layer is broken, and a potential difference occurs. The potential difference results in the formation of throughholes due to the corrosion of the barrier layer and the formation of a reaction product of a metal ion as the electrolyte called “dendrite.” These unfavorable phenomena shorten the service life of the battery.
B. Further, stacking the acid-modified polyolefin resin as the sealant layer in the armor body or the innermost layer in the sealant layer is disadvantageous, for example, in that, as compared with general polyolefin resins, the moldability or fabricability is inferior and, in addition, the cost is higher. For this reason, a method has hitherto been adopted wherein a general polyolefin resin layer is used as the sealant layer in the armor body or the innermost layer in the sealant layer and a film for a lead, which is heat bondable to both the sealant layer or the innermost layer in the sealant layer and the lead, is interposed in the lead part.
More specifically, as shown in FIG. 2G (a), a film 6′ for a lead, which has heat sealing properties on both a metal lead and a sealant layer in the armor body or the innermost layer in the sealant layer, is interposed between a lead 4 and a heat seal layer 14′ in a laminate 10′ to ensure hermetic sealing properties in the lead part.
A film of the above unsaturated carboxylic acid-grafted polyolefin, a metal-crosslinked polyethylene, or a copolymer of ethylene or propylene with acrylic acid or methacrylic acid may be used as the film for a lead.
The sealant layer or the innermost layer in the sealant layer in the laminate for constituting the armor body for a battery (hereinafter referred to as “armor body”) is formed of polypropylene, for example, from the viewpoints of heat resistance and hermetic sealing properties. In this case, a polypropylene resin, which exhibits good hermetic sealing properties and is likely to be collapsed upon heating and pressing at the time of heat sealing, that is, a polypropylene resin having a large melt index (hereinafter referred to as “MI”), is used. An acid-modified polypropylene film is used as the film for a lead. Upon hermetic heat sealing using the packaging material for a battery having the above construction and the film for a lead having the above construction, as shown in FIG. 2G (b), both the heat seal layer 14′ in the armor body and the film layer 6′ for a lead are melted, at a portion where the lead exists, by heat and pressure for heat sealing and, further, upon pressing, are often extruded to the outside of the region of the pressing part. As a result, the aluminum foil as the barrier layer 12′ in the armor body 10′ often comes into contact (S) with the metal lead 4′, resulting in short-circuiting.
Further, as shown in FIGS. 2H (a) to 2H (c), upon heat sealing of the peripheral edge of the armor body, microcracks (hereinafter referred to as “root cutting C”) often occur in the sealant layer in its portion near the inner edge of the sealed part. Upon root cutting, an electrolysis solution comes into direct contact with the barrier layer. As a result, insulation among the battery body, the metal of the lead, and the barrier layer is broken, and a potential difference occurs. The potential difference results in the formation of throughholes due to the corrosion of the barrier layer and the formation of a reaction product of a metal ion as the electrolyte called “dendrite.” These unfavorable phenomena shorten the service life of the battery.
C. Further, stacking the acid-modified polyolefin resin as the sealant in the armor body is disadvantageous, for example, in that, as compared with general polyolefin resins, the moldability or fabricability is inferior and, in addition, the cost is higher. For this reason, a method has hitherto been adopted wherein a general polyolefin resin layer is used as the sealant layer in the armor body and a film for a lead, which is heat bondable to both the sealant layer and the lead, is interposed in the lead part.
Further, in the packaging material for a battery, comprising the substrate layer, the barrier layer, the adhesive resin layer, and the sealant layer, when the sealant layer is formed of, for example, a polypropylene resin, the adhesive resin layer in the laminate of the barrier layer and the sealant layer is formed of acid-modified polypropylene.
An acid-modified polyolefin with a large MI value having good processability is used as the adhesive resin layer in the laminate for constituting the armor body for a battery (hereinafter referred to as “armor body”). The acid-modified polyolefin having a large MI value, however, is a resin which is likely to be collapsed upon exposure to heat and pressure at the time of heat sealing. Upon hermetic heat sealing using the packaging material for a battery having the construction using the above adhesive resin layer, as shown in FIG. 3G (b), all of the adhesive resin layer and the sealant layer 14′ in the armor body and the film layer 6′ for a lead are melted, at a portion where the lead exists, by heat and pressure for heat sealing and, further, upon pressing, are often extruded to the outside of the region of the pressing part. As a result, the aluminum foil as the barrier layer 12′ in the armor body 10′ often comes into contact (S) with the metal lead 4′, resulting in short-circuiting.
Further, as shown in FIGS. 3H (a) to 3H (c), upon heat sealing of the peripheral edge of the armor body, very small root cutting C often occurs in the adhesive resin layer and the sealant layer in their portion near the inner edge of the sealed part. Upon the occurrence of root cutting, an electrolysis solution comes into direct contact with the barrier layer. As a result, insulation among the battery body, the metal of the lead, and the barrier layer is broken, and a potential difference occurs. The potential difference results in the formation of throughholes due to the corrosion of the barrier layer and the formation of a reaction product of a metal ion as the electrolyte called “dendrite.” These unfavorable phenomena shorten the service life of the battery.
D. Further, a pouch-type armor body, in which a laminate is formed into a bag and a battery body is housed in the bag, or an emboss-type armor body, in which the laminate is press molded to form a concave portion and a battery body is housed in the concave portion, has been developed. As compared with the pouch type, the emboss type can provide a more compact package. In any type of armor body, for example, moisture barrier properties as a battery, strength such as piercing resistance, and insulating properties are indispensable in the armor body for a battery.
The packaging material for a battery is a laminate comprising at least a substrate layer, a barrier layer, and a sealant layer. It has been confirmed that the interlaminar bonding strength among the above layers affects properties required of the armor body of the battery. For example, unsatisfactory bonding strength between the barrier layer and the sealant layer is causative of the entry of water from the exterior. The entry of water causes the corrosion of the aluminum face by hydrofluoric acid produced by a reaction of the electrolyte in the components constituting the battery with the above water and consequently causes delamination between the barrier layer and the sealant layer. Further, in the formation of the emboss-type armor body, at the time of press molding of the laminate to form a concave portion, delamination between the substrate layer and the barrier layer often occurs.
Further, when a resin having a high tensile modulus of elasticity is used in the sealant layer, at the time of emboss molding, the sealant layer often undergoes whitening or slight cracking in its surface. Further, the molding stability is poor, and pinholes, molding wrinkling, or cracks often occur.
Furthermore, hermetical sealing properties after filling of the contents and sealing may be mentioned as properties which are indispensable as the packaging material for a battery. For example, when the seal strength of the packaging material is low, a satisfactory time is necessary for sealing in a content filling/sealing line. This significantly hinders cycle shortening and often deteriorates the production efficiency.